Methods for manufacturing multi-layer balloons for medical applications

ABSTRACT

A multi-layered balloon is provided where each layer is formed such that each layer is made from tubing that optimizes the inner wall stretch thus providing maximum balloon strength. The high pressure, multi-layer balloon is provided with layers that allow for slipping, such that the balloon has a very high pressure rating and toughness, yet excellent folding characteristics. Methods for producing such multi-layer balloons using existing balloon forming equipment are also provided. The multi-layer balloons can have alternating structural and lubricating layers, or layers with low-friction surfaces. The multi-layer balloons are preferably manufactured using a variety of methods including nesting, co-extrusion, or a combination of nesting and co-extrusion. The multi-layer balloons have balloon layers having substantially similar, or the same, high degree of biaxial orientation of their polymer molecules such that each balloon layer of the multi-layer balloon will fail at approximately the same applied pressure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/108,868 filed May 16, 2011 and currently pending, which is adivisional of U.S. patent application Ser. No. 11/611,748, filed Dec.15, 2006, which issued on May 17, 2011 as U.S. Pat. No. 7,942,847 andwhich claims priority under 35 U.S.C. §119(e) to U.S. ProvisionalApplication Ser. No. 60/751,014 filed on Dec. 16, 2005, entitled “VeryHigh Pressure Multi-Layer Balloons for Medical Applications and Methodsfor Manufacturing Same,” and to U.S. Provisional Application Ser. No.60/831,529 filed on Jul. 18, 2006, entitled “Multi-Layer Balloons forMedical Applications and Methods for Manufacturing the Same,” all ofwhich are incorporated herein by reference in their entireties.

BACKGROUND

1. Field

Embodiments of this invention relate generally to balloon catheters andmethods for making balloon catheters for medical applications. Inparticular, embodiments of this invention relate to multi-layer ballooncatheters having at least two structural layers and at least onelubricating layer that can be formed through a nesting method.

2. Description of the Related Art

An increasing number of surgical procedures involve percutaneouslyinserted devices that employ an inflatable thin wall polymer balloonattached to the distal end of a small diameter hollow shaft called acatheter. The device can be advanced to the treatment site via anartery, vein, urethra, or other available passage beneath the skin. Theshaft usually exceeds 130 cm in length so that the balloon can bepositioned deep within the patient's body. The opposite (proximal) endof the shaft, typically having an inflation connector, remains externalto the patient.

When a balloon is advanced to a treatment site, the balloon is deflatedand tightly wrapped around the shaft to minimize its cross-section andfacilitate easy insertion and navigation through the passage. Afterreaching the desired location, the balloon is slowly inflated with ahigh pressure saline solution. The balloon walls unfold and expandradially. During this process a substantial radial force can be exertedby or on the balloon walls. This hydraulically generated radial forcecan be utilized for a number of different medical procedures such as,for example, vessel dilation, stent deployment, passage occlusion, andbone compression or distraction (such as distraction of vertebrae in thespinal column).

Several factors can limit the force a balloon can exert while within apatient. For example, for a particular cross-sectional balloon size, thedesign of a balloon, the material used to construct the balloon, and thestructural integrity of a balloon can limit the force a balloon canexert without failing (e.g., bursting). Minimizing the risk of balloonbursting can be important in many medical procedures because, uponbursting, balloon debris may become lodged within a patient causingpotentially severe trauma. Additional, higher pressures may be needed toaffect the treatment.

The hydraulically generated pressure, as noted above, typically exertstwo types of stress on the balloon. Radial stress (or hoop stress)pushes a cylindrically-shaped balloon radially outward. Radial stresscan lead to axial bursting of the balloon parallel to its longitudinalaxis. Axial stress, on the other hand, pushes a cylindrically-shapedballoon axially outward. Axial stress can lead to radial bursting of theballoon somewhere along the balloon's circumference (e.g., completefracture of the balloon).

Both radial stress and axial stress have a linear relationship inpressure to the balloon's wall thickness and the ratio of the balloon'sdiameter to the balloon's wall thickness. As a result, any increase inpressure or diameter size requires an equally proportional increase inthe balloon's thickness to avoid a critical pressure level (i.e., burstpressure) that will cause the balloon to burst. Generally, radial stressis twice as large as axial stress, so balloons will frequently burstaxially absent some deformity or preprocessing. However, in the presenceof balloon deformities, a balloon may burst radially. Such a radialbursting could disadvantageously leave separated sections of the ballooninside the patient after the catheter is removed.

Increasing balloon wall thickness also increases the cross-section ofthe balloon when deflated and wrapped for insertion. Consequently, aballoon having an increased balloon wall thickness might have limitedaccess to certain areas in a patient due to the balloon's increasedsize. Typically, the balloon's stiffness varies as a cube of theballoon's thickness. For example, doubling the balloon's wall thicknessresults in doubling the burst pressure or the balloon diameter withoutbursting, but also increases the stiffness by a factor of eight. Thisadded wall stiffness impairs one's ability to tightly wrap the balloonaround the catheter shaft, which is necessary to limit the size of theballoon's cross-sectional area. If the balloon is bent too much beyondits stiffness, undesirable deformities may result. Usually, a balloonhaving a wall thickness of less than 0.0022 inches must be used to avoidthe above-mentioned problems.

Balloon deformities can be caused in many situations such as duringformation, by scratching, by stretching, or by bending. Thesedeformities lead to a concentration of stress when the balloon issubject to pressure, which can lead to further deformation andultimately a lower critical burst pressure. Scratching of the balloon bya device attached to the catheter, such as a stent, is a relativelycommon concern.

A number of techniques are being used to modify balloon properties inorder to improve balloon functionality. These techniques includeblending different types of polymers, adding plasticizers to balloons,and modifying parameters of the balloon forming process. These methodsare often not entirely successful in creating a more desirable balloonwith improved mechanical characteristics. Typically, these knowntechniques improve one balloon performance parameter while deterioratinganother parameter.

Some have attempted to resolve this problem by using multi-layerballoons. For the reasons described below, these prior art multi-layerballoons also have serious deficiencies.

SUMMARY

One aspect of embodiments of the present invention involves creatingmulti-layer balloons where each layer is made from tubing that optimizesthe inner wall stretch thus providing maximum balloon strength. Themulti-layer balloons have very high pressure ratings and toughness, yetexcellent folding characteristics. Methods for producing suchmulti-layer balloons using existing balloon forming equipment are alsoprovided.

Another aspect comprises a balloon with two structural layers having aslip layer disposed between the structural layers. The slip layeradvantageously allows sliding between adjacent layers. As a result,flexibility of the multi-layer balloon is increased over single layerballoons having an equal wall thickness. Other aspects involve adifferent number of structural layers and lubricating layers, such as,for example, three structural layers and two lubricating layers, fourstructural layers and three lubricating layers, and five structurallayers and four lubricating layers.

Another aspect involves a multi-layer balloon where each balloon layerhas the same size (e.g., diameter and/or wall thickness), is comprisedof the same material or materials having substantially identicalmechanical properties, and has the same degree of molecular orientationin the body portion of the balloon. It will be apparent that in somesituations it will be desirable to have some balloon layers havingdifferent sizes, materials, and/or degree of molecular orientations upondeflation, while at the same time having equivalent size, mechanicalproperties, and/or orientation upon inflation. For other applications,it will be apparent that one can vary size, material, and/or orientationto at least some degree while still remaining within the spirit of theinvention.

Another aspect comprises a balloon with a plurality of layers, whereinat least one structural layer has low friction surfaces. It will beapparent that further variations are possible involving differentcombinations of lubricating layers and structural layers. Theselubricating and structural layers need not be in an alternatingconfiguration.

In yet another aspect, structural layers can be polyamides, polyesters,polyethylenes, polyurethanes and their co-polymers. It will be apparentthat further variations are possible involving structural layers ofother material or chemical composition.

In one aspect of embodiments of the present invention, the layers can beadapted to the particular stresses, pressures, and deformities to whichthey might be vulnerable. For example, because the top layer might beexposed to sharp objects (such as stents, calcified plaque, bone, orother natural protrusions within a patient's body), the top layer couldbe made from a more compliant material that is scratch resistant. Theinner layers of the multi-layer balloon, which are generally not exposedto sharp objects, could be made from a less compliant material with ahigher burst strength. It will be apparent that further variations arepossible, depending on which stresses, pressures, and deformities thelayers must withstand in a particular medical application.

In another aspect, lubricating layers can be silicon oil, “bucky balls”(carbon nanopowder), high-density polyethylene, tetrafluoroethylene, ora mixture thereof. It will be apparent that further variations arepossible involving lubricating layers of other material or chemicalcomposition.

Another aspect involves a method for creating multi-layer balloons withlow friction interfaces by nesting multiple balloons or by nestingco-extruded tubing. It will be apparent that these methods can becombined with each other and other balloon forming methods to producelarger multi-layer balloons.

In one aspect, the bodies of the balloons can be extruded separately onthe same mold to ensure that they have equivalent, or substantiallyequivalent, size. The necks, however, might need to be different sizesto ensure optimal welding and/or attachment to the catheter. It will beapparent that other methods can be used to obtain approximatelyequivalent sized balloons. It will also be apparent that similar resultscan be achieved by making the outer balloon wider than the innerballoon.

In another aspect, separately formed balloons can be nested afteraltering the orientation of one balloon to make it thinner, facilitatinginsertion. One way to accomplish this is by axial stretching. It will beapparent that other methods can be used to make a balloon thinner.

In another aspect, already nested balloons can be heated, stretched, andinflated simultaneously to achieve optimal molecular alignment. It willbe apparent that this need not be done simultaneously, especially whennesting can be done after the balloons are heated, stretched, andinflated to equivalent size and orientation. Similarly, it will beapparent that the balloons need not be formed and processed identicallyto obtain equivalent burst strengths, sizes, and/or molecularorientations. This is especially true for balloons of differentmaterials. Other suitable methods can also be used to achieve uniformmolecular alignment among the balloon layers.

In yet another aspect, lubricant can be added at any stage of themulti-layer balloon forming process. The lubricant can be co-extrudedonto or between balloon layers, applied to balloon layers afterextrusion but before nesting, or injected between balloon layers afternesting. In one embodiment, lubricant can be kept separate from certainregions of the balloon. This can be valuable to promote friction in thatarea if desired. This can also be valuable if the lubricant interfereswith welding the balloon layers to each other or to the catheter. Inanother embodiment, lubricant can be distributed between the balloonlayers before or after balloon welding. It will be apparent that thiscan be accomplished under a wide variety of methods.

In another aspect of embodiments of the present invention, alreadynested or co-extruded balloons can be treated as a single balloon in thecontext of this invention. As a result, one can manufacture balloonswith a greater numbers of layers than those specifically disclosedherein.

In another aspect of embodiments of the present invention, tubing forthe outer balloon can be co-extruded with a lubricious layer on itsinside wall. Tubing for the inner balloon, which would not possess alubricious layer, can be stretch longitudinally to fit within the tubefor the outer balloon. This nested tube arrangement can then be used toblow a balloon in a single process. Note that longitudinal stretch doesnot affect the tubing's radial stretch. This embodiment is an importantconsideration because trying to longitudinally stretch a tube with aco-extruded lubricious layer, such as by stretching a tube with alubricious outer layer to nest within another tube, would result insagging or separation of the lubricious layer.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will now be described in connection with preferred embodimentsof the invention shown in the accompanying drawings. The illustratedembodiments, however, are merely an example and are not intended tolimit the invention. The drawings include twenty-five figures, which arebriefly described as follows:

FIG. 1A is a perspective view of an exemplary prior art ballooncatheter.

FIG. 1B is an enlarged perspective view of a cross-section of a priorart balloon catheter shaft.

FIG. 2 is a perspective view of a balloon catheter having a plurality offlutes.

FIG. 3A is a cross-sectional view of a fluted balloon catheter beforewrapping has been performed.

FIG. 3B is a cross-sectional view of a fluted balloon catheter afterwrapping.

FIGS. 3C through 3E are enlarged cross-sectional views of threedifferent fluted balloon catheters after wrapping.

FIG. 3F is an enlarged cross-sectional view of a fluted balloon catheterafter wrapping and compression.

FIG. 4 is an enlarged cross-sectional view of a fluted balloon catheterthat has developed a crack deformity upon wrapping.

FIG. 5 is a perspective view of a balloon catheter that has developed ascratch deformity.

FIG. 6 is a perspective view of a balloon catheter that has developed acat-eye deformity.

FIG. 7A is an enlarged cross-sectional view of a fluted multi-layerballoon catheter after wrapping.

FIG. 7B is an enlarged cross-sectional view of a fluted single layerballoon catheter after wrapping.

FIG. 8A is a cross-sectional view of a multi-layer balloon catheterafter inflation.

FIG. 8B is a cross-sectional view of a single layer balloon catheterafter inflation.

FIG. 9 is a schematic showing the stretching of polymers to align theirmolecular chains through a blow molding process.

FIG. 10 is a stress-strain curve with strain, or the amount that aballoon will stretch, on the x-axis and stress, or the applied pressure,on the y-axis. FIG. 10 shows that once optimal stretch is achieved, aballoon material will have its greatest strength and will resist furthergrowth.

FIG. 11 is a diagram illustrating the inner diameter stretch and theouter diameter stretch of single-layer balloon tubing when expanded andshowing that the outer diameter stretch is less than the inner diameterstretch.

FIG. 12 is a stress-strain curve showing that when the inner wallstretch of single-layer balloon tubing is optimized, the outer wallstretch is sub-optimal and will continue to expand when applied pressureis increased.

FIG. 13 is a diagram illustrating the inner and outer radii ofsingle-layer balloon tubing in an unexpanded and an expanded state.

FIG. 14 is a graph showing single-layer balloon catheters havingdiameters of 2 mm, 4 mm, and 6 mm, with wall thickness on the x-axis andthe ratio of inner wall stretch to outer wall stretch on the y-axis.

FIG. 15 is a schematic showing the wall profile of a single-layerballoon catheter that is represented in the graph of FIG. 16.

FIG. 16 is a graph of a single-layer balloon catheter showing therelative stretch ratio as a function of wall slice with wall position onthe x-axis and percentage of inner balloon stretch on the y-axis.

FIG. 17 is a graph of a single-layer balloon catheter and a two-layerballoon catheter manufactured from co-extruded tubing. FIG. 17 shows theinner stretch of wall slices of the two-layer balloon relative to theinner stretch of corresponding wall slices of the single-layer balloon.

FIG. 18 is a graph of a single-layer balloon catheter and two-layerballoon catheter manufactured from tubing in which the inner wallstretch has been optimized for maximum strength. FIG. 18 shows the innerstretch of wall slices of each layer of the two-layer balloon relativeto the inner stretch of corresponding wall slices of the single-layerballoon.

FIG. 19A is a perspective view of a balloon catheter having an elementshown aligned in a longitudinal direction and in a lateral direction.

FIG. 19B is an enlarged perspective view of the longitudinally-alignedelement of the balloon catheter as shown in FIG. 19A.

FIG. 20A is a diagram of a single layer element with a small thicknessbending like a cantilevered beam shown with an applied force and amaximum deflection.

FIG. 20B is a diagram of a single layer element with a large thicknessbending like a cantilevered beam shown with an applied force and amaximum deflection.

FIG. 20C is a diagram of a multi-layer element with three layers eachhaving small thicknesses bending like a cantilevered beam shown with anapplied force and a maximum deflection.

FIG. 20D is an enlarged side elevational view of the multi-layer elementshown in FIG. 20C.

FIG. 21 is a cross-sectional view of a portion of a multi-layer balloonhaving a discontinuous lubricating layer.

FIG. 22A is a side elevational view of an inner balloon used in a methodfor nesting balloons to form a multi-layer balloon.

FIG. 22B is a side elevational view of the inner balloon after heatingand stretching of the method for nesting multi-layer balloons of FIG.22A.

FIG. 22C is a side elevational view of the inner balloon after flutingof the method for nesting multi-layer balloons of FIG. 22A.

FIG. 22D is a side elevational view of the heated, stretched, and flutedinner balloon and an outer balloon used in the method for nestingmulti-layer balloons of FIG. 22A.

FIG. 22E is a side elevational view of a multi-layer balloon wherelubrication is being applied between the inner balloon and the outerballoon of the method for nesting multi-layer balloons of FIG. 22D.

FIG. 22F is a side elevational view of the multi-layer balloon afterheating, stretching, and inflating so that the inner balloon and theouter balloon have the same, or a substantially similar, degree ofmolecular alignment of the method for nesting multi-layer balloons ofFIG. 22D.

FIG. 22G is a side elevational view of the multi-layer balloon afterfluting of the method for nesting multi-layer balloons of FIG. 22D.

FIG. 23A is a side elevational view of a three layer balloon and a twolayer balloon used in a method for co-extruding balloons to form amulti-layer balloon.

FIG. 23B is a side elevational view of the three layer balloon afterheating and stretching so as to the decrease the diameter of the threelayer balloon prior to insertion into the two layer balloon of themethod for co-extruding multi-layer balloons of FIG. 23A.

FIG. 23C is a side elevational view of the three layer balloon having adecreased diameter being inserted into the two layer balloon having itsoriginal diameter of the method for co-extruding multi-layer balloons ofFIG. 23A.

FIG. 23D is a side elevational view of a multi-layer balloon having fivelayers after heating, stretching, and inflating so that the three layerballoon component and the two layer balloon component have the same, ora substantially similar, degree of molecular alignment of the method forco-extruding multi-layer balloons of FIG. 23A.

FIG. 24A is a side elevational view of a multi-layer balloon formedusing the methods disclosed showing a method for welding the necks ofthe multi-layer balloon in order to securely attach the balloon layersto each other.

FIG. 24B is a side elevational view of the multi-layer balloon havingits necks welded of FIG. 24A.

FIG. 25A is a side elevational view of a single-layer tubular extrusionwithout a slip layer and a single-layer tubular extrusion of the samesize having a slip layer on its inner surface used in a method to form atwo-layer high pressure balloon.

FIG. 25B is a side elevational view of the single-layer tubing without aslip layer after axial stretching to decrease its diameter prior toinsertion into the single-layer extrusion having a slip layer on itsinner surface in the method for nesting two-layer balloons of FIG. 25A.

FIG. 25C is a side elevational view of the single-layer extrusion havinga decreased diameter being inserted into the single-layer extrusionhaving its original diameter in the method for nesting two-layerballoons of FIG. 25A.

FIG. 25D is a side elevational view of a two-layer parison comprising afirst balloon layer, a slip layer, and a second balloon layer such thatthe first balloon layer and the second balloon layer have the same, or asubstantially similar, degree of molecular alignment in the method fornesting two-layer balloons of FIG. 25A.

DETAILED DESCRIPTION

Embodiments of the present invention will now be described more fullyhereinafter with reference to accompanying drawings, in which preferredembodiments are shown. This invention may, however, be embodied in manydifferent forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and exemplary of the scope of theinvention to those skilled in the art.

FIGS. 1A and 1B show an exemplary embodiment of a prior art ballooncatheter system 1. A balloon 2 is attached to the distal end of acatheter shaft 3 and is inflated through an inflation lumen 4. A guidewire lumen 5 is provided on the catheter system 1, which allows forexternal control of the balloon 2 and the catheter 3 when the system 1is disposed within a patient. It should be noted that further variations(e.g., rapid exchange, concentric lumen, etc.) are possible for thisstructure.

FIG. 2 illustrates a perspective view of an embodiment of a prior artcatheter balloon 2 in an unwrapped and deflated configuration. Theballoon 2 is folded into a plurality of flutes 6, typically ranging fromthree to eight flutes. The plurality of flutes 6 are formed in adirection substantially parallel to a longitudinal direction of theballoon 7. The plurality of flutes 6 are folded with a slight curvaturein order to facilitate subsequently wrapping the fluted balloon 2 aroundthe catheter shaft 3 (as shown in FIG. 1A). The balloon 2 attaches tothe catheter shaft 3 both at a proximal neck of the balloon 50 and at adistal neck of the balloon 51. The balloon 2 also includes a bodyportion 52, which can be inflated and deflated when the balloon 2 isdisposed within the body of a patient during a particular medicalprocedure.

FIG. 3A shows a cross-section of an embodiment of a prior art flutedballoon 2 on a catheter shaft 3. The fluted balloon 2 has a plurality offlutes 6. In the illustrated embodiment, the plurality of flutes 6comprises six flutes. The deflated fluted balloon 2 has a relativelysmall cross-sectional area, but can have a relatively wide diameterbecause the thin flutes 6 stretch radially outward from the cathetershaft 3. Upon inflation, the balloon 2 can expand to have a much largerdiameter and cross-sectional area 8, as shown in the circular phantomlines in FIG. 3A.

FIG. 3B shows a cross-section of an embodiment of a prior art flutedballoon 2 after it has been wrapped. The plurality of flutes 6 arefolded down and about the catheter shaft 3 such that they are in closecontact with each other and the catheter shaft 3. Once the balloon 2 iswrapped, the deflated balloon's diameter and cross-sectional area 9(sometimes referred to as the crossing profile) is much smaller than theinflated balloon's diameter and cross-sectional area 8 (as seen in thecircular phantom lines in FIG. 3B). Having a balloon 2 with a smalldiameter and cross-sectional area 9 allows the catheter 2 to be guidedthrough smaller passageways within a patient's body. Inflating theballoon 2 to have a larger diameter and cross-sectional area 8advantageously allows for the placement of a larger stent, occlusion ofa larger passageway, and generally greater versatility once the catheter2 has reached a particular treatment site within a patient's body.

FIGS. 3C through 3E generally illustrate enlarged views of severalconfigurations of balloon folding patterns. FIG. 3C illustrates anenlarged side elevational view of a cross-section of a prior art flutedballoon 2 c after wrapping. As shown in FIG. 3C, the reduction in sizeof the wrapped balloon 2 c about the catheter shaft 3 is limited by theballoon's bend radius 10 c. In general, a balloon's bend radiusincreases with the thickness and toughness of the balloon, as can beseen by comparing FIG. 3C with FIGS. 3D and 3E. FIG. 3D shows a balloon2 d that is thicker than the balloon 2 c shown in FIG. 3C. As can beseen in FIG. 3D, the bend radius 10 d for the thicker balloon 2 d islarger than the bend radius of the balloon 2 c in FIG. 3C. FIG. 3E showsa balloon 2 e having the same thickness as the balloon 2 c of FIG. 3C,but being composed of a tougher material than that of the balloon inFIG. 3C. As can be seen in FIG. 3E, the bend radius 10 e for the tougherballoon 2 e is also larger than the bend radius of the balloon 2 c inFIG. 3C. Accordingly, both a thicker balloon 2 d and a tougher balloon 2e typically cannot be folded into as small a cross-section as theballoon 2 c of FIG. 3C. The bend radius of a balloon is importantbecause bending a balloon beyond its bend radius can cause deformitieswhich will lower the balloon's resistance to bursting when inflated.

FIG. 3F shows a balloon 2 f wrapped about a catheter shaft 3. Theballoon 2 f has a negligible bend radius and can, therefore, be tightlywrapped about the catheter shaft 3 without any protrusions developing onthe outer surface of the folded and wrapped balloon 2 f Advantageously,this configuration permits the diameter and the cross-section of theballoon 2 f to be minimized prior to, and during, insertion of theballoon catheter system into a patient's body. In addition, as discussedin further detail below, this configuration minimizes failure of theballoon 2 f during a medical application due to a deformity developingon the balloon's outer surface.

FIGS. 4 through 6 generally show deformities that can develop on aballoon's outer surface. As shown in FIG. 4, a wrapped balloon 2 isfolded and compressed beyond its bend radius 10 creating a crack 11 inthe outer surface of the wrapped balloon 2 near the site of a fold. Suchcracking is more likely for less compliant materials, which alsogenerally have higher burst strengths. Thus, there is a general tradeoff between burst strength and flexibility. Once the crack 11 hasformed, stress will concentrate near the crack 11 when the balloon 2 isinflated, causing the crack 11 to expand and ultimately causing failureof the balloon 2 (e.g., by bursting).

FIG. 5 shows another deformity that occurs in balloons. When a medicaldevice such as a stent is applied over a balloon 2, it can create ascratch 12. The scratch 12 generally extends in the longitudinaldirection of the balloon 2. Again, the likelihood of scratching can beminimized by using a more compliant material, which also has a lowerburst strength. Once the scratch 12 has formed, stress will concentratenear the scratch 12 when the balloon 2 is inflated, causing the scratch12 to expand and ultimately causing failure of the balloon 2 (e.g., bybursting).

FIG. 6 illustrates yet another type of deformity. When a balloon isformed, there may be regions of low molecular density or imperfectionsin the molecular lattice. As a result, a small hole 13 can form uponstretching the balloon 2. The hole 13 can grow as the balloon 2 isstretched further, often resembling a “cat-eye.” Stress concentratesnear the edges of the cat-eye deformity 13. Since the balloon 2 isstretched during inflation, this can also lead to failure of the balloon2 (e.g., by bursting).

FIGS. 7A and 8A show an enlarged cross-section of an embodiment of amulti-layer balloon 2 having a first layer 20, a second layer 22, and athird layer 24. In one embodiment, in which the multi-layer balloon 2comprises a balloon having three structural layers, the first layer 20comprises a top layer of the multi-layer balloon, the second layer 22comprises a middle layer of the multi-layer balloon, and the third layer24 comprises a bottom layer of the multi-layer balloon. The multi-layerballoon 2 is shown in the wrapped position, similar to positionillustrated in FIG. 3C. The first layer 20 of the multi-layer balloonhas a thickness that is approximately one-third the thickness of thesingle-layer balloon shown in FIG. 3C. The second layer 22 and the thirdlayer 24 also each have a thickness that is approximately one-third thethickness of the single-layer balloon shown in FIG. 3C. Because eachlayer 20, 22, 24 is thinner than the single-layer balloon of FIG. 3C,the bend radius 10 is smaller for an equal cumulative thickness 3t.Because the cumulative thickness of the multi-layer balloon 2 of FIG. 7Ais substantially the same as the thickness of the single-layer balloonof FIG. 3C, the burst pressure P will also be the substantially the sameas long as adjacent balloon layers of the multi-layer balloon can sliderelative to each other.

As shown in FIGS. 7B and 8B, a balloon 2 with a single layer has a totalthickness 3t that is equivalent the thickness of the multi-layer balloon2 shown in FIGS. 7A and 8A. As shown in FIG. 7B, the thicker balloon 2has a larger bend radius 10, and thus cannot be folded as closely to thecatheter shaft 3. If a scratch develops on the first layer 20 of themulti-layer balloon during the crimping and wrapping process, the firstlayer 20 could burst while, at the same time, the other balloon layers22, 24 retain their structural integrity. More generally, a singleballoon layer 20, 22, 24 might fail as a result of a deformity, such asthose shown in FIGS. 4 through 6, on any layer 20, 22, 24. As a result,the multi-layer design provides redundancy that could be valuable incertain medical procedures. Furthermore, because the multi-layer designis more flexible, as discussed below, deformities as shown in FIG. 4 areless likely to occur. Meanwhile, the burst pressure P for a multi-layerballoon is substantially the same as that for an equivalent thicknesssingle layer balloon, as can be seen by comparing FIG. 8A with FIG. 8B.It will be apparent that similar effects can be achieved by varying thematerial in each balloon layer, varying the number of balloon layers,and varying other aspects of this embodiment.

In one embodiment, the first layer 20 of the multi-layer balloon is madeof a soft material that is preferably scratch and puncture resistant.When a device such as a stent is applied to the catheter system, it istypically crimped onto the balloon 2. The applied crimping force shouldbe such as to provide a sufficiently strong attachment force, yet itshould also not scratch, pierce, or otherwise damage the balloon wall.By using a softer first layer 20 (which can comprise an outer layer ofthe multi-layer balloon), the risk of failure due to scratching can bedecreased.

The second layer 22 and the third layer 24 (which can comprise innerlayers of the multi-layer balloon) can be made of a tougher materialthat is less scratch resistant, but able to withstand higher appliedpressures. These layers 22, 24 can be protected from scratching by thesoft outer layer 20, but still can provide additional strength to themulti-layer balloon. It should be noted that the above-described effectsneed not always be achieved simultaneously, and they are not necessarilysensitive to the number of layers, composition of other layers, form ofdevice carried by the catheter, or other aspects of this embodiment.

As is discussed in greater detail below, each layer 20, 22, 24 may beequally sized and shaped in the body portion 52, in order to optimizethe burst characteristics of the balloon in accordance with the presentinvention. As the balloon is inflated, each layer is stretched, causingthe thickness to shrink. This causes the third balloon layer 24 tostretch approximately as far as the first balloon layer 20. If the thirdballoon layer 24 begins with a smaller diameter than that of the firstlayer 20, then the third layer 24 must stretch an additional amount tomatch the size of the first balloon layer 20. This can cause the innerballoon layers 22, 24 to burst before the outer balloon layer 20, whichcan limit the multi-layer balloon's maximum inflation to a level thatinflates the larger outer balloon layer 20 below its optimal inflationlevel. Consequently, using substantially identical balloons for eachlayer of the multi-layer balloon makes each balloon layer have asubstantially similar burst pressure, ensuring that they burstsubstantially simultaneously and reducing the possibility of sub-optimalinflation of any layer 20, 22, 24 of the multi-layer balloon. It will beapparent that balloons of different material may require different sizesand shapes to achieve this effect. It will also be apparent that,because the balloons still do not stretch to exactly equal diametersupon inflation, it may be practical to make the inner balloons slightlysmaller such that each layer stretches to substantially near its optimalinflation level.

One general problem with multi-layer balloons is that the interiorballoon layer often bursts before the exterior balloon layer. Thisoccurs because the outer layers have not been optimized for maximum wallstrength.

The interior balloon layer bursts prior to exterior balloon layersbecause the multi-layer balloon does not comprise layers having uniformburst strengths. This is primarily a result of not taking into accountthe confounding effect of radial expansion on achieving optimal radialstretch during the balloon blow molding process.

With reference to FIGS. 9 and 10, an objective of blow molding inballoon formation is to stretch the polymer material in order to achievemaximum strength and semi-compliance. This is done by aligning themolecular chains as shown in FIG. 9. During the stretching process, thematerial will grow until the polymer chains are aligned. Once thepolymer chains are aligned, the material resists further growth andprovides maximum strength. This is shown on the idealized stress-straincurve in FIG. 10. In response to the strain caused by stretching, thematerial exhibits relatively even stress. Once the polymer chains arealigned, the material resists further growth as shown by an increase instress. In the ideal cases, all polymer chains will be uniformlystretched. Various polymer materials will have different ideal stretchratios in order to achieve uniform molecular alignment.

Optimum stretch for a multi-layer, high-pressure, balloon is dependentupon a number of variables. For a given material, there is a calculatedoptimum stretch that provides optimum strength of the multi-layerballoon. The calculated optimum stretch is dependent upon, for example,the diameter of the balloon and the thickness of the layers whichcomprise the multi-layer, high-pressure, balloon. Practically, it isvery difficult to stretch a balloon to its exact optimum stretch. Thus,for most applications, stretching a material to within 15% of itsoptimum stretch, and preferably to within less than 10%, will provideoptimum balloon strength.

During the balloon forming process, the polymer material is stretchedboth radially and longitudinally in order to achieve biaxial orientationof the polymer chains. However, radial stress is twice that oflongitudinal stress. As a result, optimizing the radial stretch is moreimportant to burst resistance than longitudinal stretch.

With reference to FIGS. 11 and 12, radial stretch confounds the goal toachieve a uniform stretch of the polymer material. The reason for thisis that balloons are blow molded from tubing having thicker walls. Asshown in FIG. 11, because of the difference in wall thickness thestretch of the inner wall of the initial tubing to that of the balloonwill be greater than that of the respective outer wall stretch. In viewof the non-uniform stretch between the inner wall and the outer wall ofthe tubing, a problem encountered in the art is optimizing the radialstretch of the balloon tubing. If the outer wall stretch be optimized,then the inner wall becomes over-stretched. Consequently, the inner wallwill develop micro-tears which can lead to premature failure of theballoon tubing. Therefore, a feasible solution to this problem is tooptimize the radial stretch based on the inner wall rather than theouter wall.

As shown in the stress-strain curve in FIG. 12, the outer wall isunder-stretched when optimizing radial stretch based upon the inner wallof the balloon. When the inner wall achieves optimal alignment of itspolymer chains, as shown on the stress-strain curve, the outer wall hasnot yet reached optimal alignment of its polymer chains, as shown bybeing further down the stress-strain curve of FIG. 12. If the innerportion of the balloon wall fails, the outer portion will continue tostretch thus providing no additional strength to the balloon wall.

The relative under-stretching of the outer wall can be substantial. Thiscan be shown using a mathematical model relating the radial expansion ofa smaller-diameter hollow cylinder with a given wall thickness (theinitial extruded tube) to a hollow cylinder with a larger diameter andthinner walls (the blow molded balloon body). FIG. 13 shows the variousradii to be taken into account from a cross section of the tube andballoon. Of particular interest will be the inner wall stretch(S_(i)=R_(i)/r₁) and the outer wall stretch (S_(i)=R_(i)/r_(o)). As S,is given as being the optimized radial stretch, the relative ratio ofS_(o)/S_(i) will used to demonstrate the confounding effect of radialstretch on uniform wall strength.

Formula I, set forth below, shows the equation for the mass (M) of ahollow cylinder based on its radius (r), length (L) and density (ρ). Inexpanding the hollow cylinder represented by the tube to a balloon, themass remains the same. Accordingly, there is a fixed relationshipbetween the radii of tube to that of the balloon as shown in Formula II(the parameters with the subscripted t refers to the tubing and thesubscripted B refers to the balloon). Thus, for a balloon of a givendiameter (R_(o)/2) and wall thickness (W_(b)) with an optimized innerwall stretch, there is a specific tube size that must be used as astarting condition.

M=π(r _(o) ² −r _(i) ²)Lρ  I.

π(r _(o) ² −r _(i) ²)L _(t)ρ_(t)=π(R _(o) ² −R _(i) ²)L _(B)ρ_(B)  II.

For a given balloon, the required inner radius for the tubing is simplythe balloon outer radius less the wall thickness divided by the optimalstretch for the polymer used: r_(i)=(R_(o)−W_(b))/S_(i). Determining theouter tubing radius, r_(o), is more complicated but can be derived fromthe equation in Formula II.

As set forth below, Formula III shows such a derivation with S_(L) beingused to express the longitudinal stretch (S_(L)=L_(B)/L) and p therelative change in density (ρ=ρ_(B)/ρ_(t)). With these two equations,S_(o) and S_(i) can be calculated and the confounding effect of radialstretch shown.

r _(o)=√{square root over (S _(L)ρ(2R _(o) W _(B) −W _(B) ²)+(R _(o) −W_(B))² /S _(i) ²)}{square root over (S _(L)ρ(2R _(o) W _(B) −W _(B)²)+(R _(o) −W _(B))² /S _(i) ²)}  III.

FIG. 14 shows the ratio of S_(o)/S_(i) as a function of wall thicknessfor a number of different balloon diameters. As can be seen, therelative under-stretching of the outer wall can be substantial. Forexample, the outer wall for a 2 mm balloon with a wall thickness of0.001 inches has been stretched less than 40% relative to the innerwall. Any increase in wall thickness to try to strengthen the wall showsa further decrease in relative stretching. The same 2 mm balloon with a0.002 inch wall thickness shows a relative wall stretch of less than30%.

Turning now to FIGS. 15 and 16, the confounding effect of radial stretchcan be shown in more detail by examining the distribution of relativestretch within the wall. This can be done by “mapping” the respectivewall slice in the tube to that of the balloon. FIG. 15 shows such a mapin which the inner wall has a position of 0% and the outer wall has aposition of 100%. By calculating the stretch of a slice for the tubewall, for example the 20% line, to the equivalent slice in the balloon,the distribution of relative radial stretch can be shown. FIG. 16 showsa graph of a representative balloon with the relative stretch ratio as afunction of wall slice. As can be seen, the fall off in relative stretchis not proportional and in fact falls off more quickly from the innerwall.

The problem of inner balloon bursting is particularly common forco-extruded multi-layer balloons because the interior balloonnecessarily has a more optimized inner wall stretch compared to that ofouter layers. This is shown in detail on FIG. 17, in which the relativestretch of the wall slices of a dual layer balloon made from co-extrudedtubing is shown relative to a single wall balloon having the sameoverall wall thickness. Known methods of creating multi-layer balloonsprimarily focus on co-extruding balloon elements in order to create amulti-layer balloon. Known methods do not typically involve nestingballoons nor has the confounding effect of radial stretch beenconsidered. Even so, in the case of nesting balloons, the interiorballoon occasionally is made smaller to facilitate insertion into theexterior balloon, so the problem of varying stretching remains.

In accordance with embodiments of the present invention, in order tosubstantially increase the overall wall strength of a multi-layerballoon, each balloon layer is molded from tubing in which in the innerwall stretch has been optimized for maximum strength. FIG. 18 shows therelative stretch of wall slices for such a balloon having two layers. Ascan been seen, the relative amount of optimally stretched material isgreater than that afforded by co-extrusion.

Using this design, it is not necessary that the layers be made from thesame material or have the same wall thickness. Each layer is made suchthat the inner wall has been stretched for maximum strength, with thestretch ratio specific for that particular material. As described above,the inner wall should be stretched to within about 15% of its optimalstretch and, in some applications, preferably to within less than 10% ofits optimal stretch. As the wall strengths are additive, the burstpressure will be higher than that for any individual layer. Once theburst pressure is reached, all layers will fail. The compliancecharacteristics for the layers will preferably be equivalent.

FIGS. 19A and 19B illustrate a balloon wall element 14 of a multi-layerballoon catheter 2. To maintain flexibility in each balloon layer 20,22, 24, friction between these layers must be minimized. To illustratethis point we consider a balloon wall element 14. This element 14 has athickness t equal to that of the balloon 2, or balloon layers 20, 22,24, and a small width b and a length l. The element 14 can be configuredeither axially or radially. Taking one end of the element 14 as fixed,the element 14 can be viewed as a cantilevered beam for analyticalpurposes, as described below in FIGS. 20A through 20D.

FIG. 20A shows the balloon element 14 as a single layer of thickness t.A balloon element 14 with thickness t requires a force F₁ to bend theelement 14 a set distance y. FIG. 20B shows the balloon wall element 14as a single layer of thickness 3t. This thicker element 14 requires aforce F₂, which is twenty-seven times larger than F₁, to bend theelement 14 the same distance y as the element 14 in FIG. 20A (that is,because the force required varies as a cube of the element thickness).FIG. 20C shows a multi-layer element 14 comprised of a first element 15,a second element 16, and a third element 17. Each of the elements 15,16, and 17 has an individual thickness t. As a result, the multi-layerballoon element 14 has a cumulative thickness 3t. Each sub-element 15,16, and 17 is individually as thick as the balloon element 14 in FIG.20A, but collectively as thick as the balloon element 14 in FIG. 20B.Each individual element in FIG. 20C requires a force F₁ to bend a singleballoon element a given distance y. Collectively, the multi-layerballoon element 14 requires a force F₃ to bend the element 14 a givendistance y, which is three times as large as the force in FIG. 20A, butonly one third as large as the force in FIG. 20B. As shown in FIG. 20C,each balloon element layer 15, 16, and 17 preferably slides relative tothe other layers a distance Δl. If the balloon element layers 15, 16,and 17 are not permitted to slide, then the multi-layered balloon 14will likely be equivalent to the equally thick balloon in FIG. 20B.

Referring now to FIG. 20D, because the layers 15, 16, and 17 are inclose contact with each other and there is a potentially strong forcepushing them together, frictional effects can be very significant andprevent sliding between the layers. To minimize friction betweenadjacent layers and to allow sliding, lubricating layers 18, 19 can beadded in between structural layer 15, 16, and 17. The lubricating layers18, 19 can be made of any suitable substance, nonexclusively includinghigh density polyethylene, silicon oil, and carbon nanopowder, but inmany medical applications should be biocompatible. It should be notedthat lubricating layers are not necessary when friction betweenstructural layers is allowable and, in some applications, desirable.

With reference to FIG. 21, in some embodiments, lubricant should bedistributed so as to substantially cover the entire surface area betweenadjacent balloon layers. The consequences of not having lubricantcovering substantially the entire surface area are demonstrated in FIG.21. Between adjacent balloon element layers 15, 16 there are two gaps60, 62 shown in a lubricious layer 18. With the balloon inflated, thispotentially creates substantial friction at the gaps 60, 62. Thus, anabnormally loose region 64 can form between the gaps 60, 62 withabnormally stretched regions adjacent the loose region 64. This unequaldistribution of stress can cause a multi-layer balloon to burstprematurely. In some situations, spreading lubricant will be less of aconcern. For example, low pressure applications and balloon regions withlow stress may not require uniform spreading of a lubricious layerbetween adjacent balloon layers. It should be note that similar problemscan develop between any two adjacent balloon layers if lubricant is notevenly distributed.

Embodiments of the multi-layer balloon disclosed herein can provide asignificant performance improvement over current high pressure balloons.The disclosed embodiments allow for balloon catheters to be used in newapplications. For example, multi-layer balloons can be used in ultrahigh pressure applications such as 50 atmospheres or more for up to 10mm diameter balloons, and for high pressure applications for very largeballoons such as 12 atmospheres or more for up to 30 mm diameterballoons. The advantages provided by the multi-layer balloons disclosedherein can be attributed, at least in part, to forming each layer fromtubing where the inner wall stretch has been optimized for maximumstrength.

FIGS. 22A through 22G generally depict a method for nesting balloons toform a multi-layer balloon. As shown in FIG. 22A, an inner balloon 30 isprovided having a proximal neck 50A and a distal neck 51A. The innerballoon 30 is then heated and stretched so that the diameter andcross-sectional area of the inner balloon 30 is decreased, while thelength of the inner balloon 30 is at least partially increased, as shownin FIG. 22B. Heating and stretching the inner balloon 30 in this mannertypically alters the alignment of the polymer molecules comprising thebody of the balloon 30. The inner balloon 30 is then fluted using knownfluting methods so that the balloon 30 comprises a plurality of flutes.The inner balloon 30 is then wrapped about a catheter shaft. The flutedand wrapped inner balloon 30 is illustrated in FIG. 22C. The balloon 30can be fluted and wrapped, for example, using known fluting and wrappingmachines. Embodiments of such machines can be found in U.S. patentapplication Ser. No. 11/303,546, filed Dec. 16, 2005 and entitled“Balloon Catheter Folding and Wrapping Devices and Methods,” thecontents of which are hereby incorporated by reference in theirentirety. Other suitable balloon fluting and wrapping devices, however,can also be used.

With reference to FIG. 22D, the fluted and wrapped inner balloon 30 canbe inserted into an outer balloon 31. The outer balloon 31 preferablyhas properties that are substantially similar, or in some casesidentical, to those properties of the unstretched and unheated innerballoon 30 described with reference to FIG. 22A. In one embodiment, theballoons 30, 31 are comprised of tube stock that optimizes the innerwall stretch of the balloons 30, 31.

The outer balloon 31 has a proximal neck 50B and a distal neck 51B. Inone embodiment, the proximal neck 50B and the distal neck 51B of theouter balloon 31 have larger diameters than the proximal neck 50A anddistal neck 51A of the inner balloon 30. In one embodiment, the innerballoon 30 can be inserted into the outer balloon 31 by drawing itthrough the outer balloon 31 such that the inner balloon 30 issubstantially contained within the outer balloon 31. Other suitablemethods can also be used to insert the inner balloon 30 into the outerballoon 31.

In one embodiment of the present multi-layer balloon nesting method, theinner balloon 30 and the outer balloon 31 are blow-molded on the samemold (but preferably at separate times) so that the balloons 30, 31 havea substantially similar shape and size along a body portion of theballoons 30, 31. In this embodiment, the balloons 30, 31 preferably haveproximal and distal necks having different sizes, as illustrated inFIGS. 22A and 22D. That is, the proximal and distal necks 50A, 51A ofthe inner balloon 30 have a smaller diameter than the proximal anddistal necks 50B, 51B of the outer balloon 31.

With reference to FIG. 22E, once the inner balloon 30 has been insertedinto a cavity of the outer balloon 31, lubrication 32 can be added to aspace disposed between the inner balloon 30 and the outer balloon 31. Inone embodiment, the lubrication 32 comprises silicon oil. It should benoted that lubrication 32 can be added either before the insertion stepshown in FIG. 22D, during the insertion step shown in FIG. 22D, or afterthe insertion step shown in FIG. 22D. In the illustrated balloon nestingmethod, as shown in FIG. 22E, lubrication 32 is added after theinsertion step in FIG. 22D.

As shown in FIG. 22F, the nested balloons 30, 31 are next heated,stretched, and inflated to bring the respective body portions of theinner balloon 30 and the outer balloon 31 into the same, or asubstantially similar, molecular alignment. Embodiments of devicescapable of inflating and heating a balloon can be found in U.S. patentapplication Ser. No. 11/303,545, filed Dec. 16, 2005 and entitled“Measurement Apparatus and Methods for Balloon Catheters,” the contentsof which are hereby incorporated by reference in its entirety. Theembodiments presented can be modified to stretch the balloon as well,and also can be used to verify that the balloons have been stretched toan optimal size and shape. Other embodiments can be used to heat,stretch, and inflate the multi-layer balloons disclosed herein.

In one embodiment of the nesting method, one can heat and stretch theballoon and then begin inflating the balloon while continuing to heatand stretch the balloon. Inflation of the balloon can commence whenapproximately thirty percent of the stretching remains to be completed.The balloons are preferably stretched to four to five times theirinitial length. This amount of stretching is meant to optimize biaxialmolecular alignment, and it will be apparent that a different methodwill be suitable for different applications.

With continued reference to FIG. 22F, a containing apparatus 61 can beused to prevent the lubrication 32 from reaching a welding zone 40.After sealing the balloons 30, 31, a lubricating layer 32 can bedistributed evenly by mechanical means, if it is not sufficientlydistributed during inflation.

As illustrated in FIG. 22G, the multi-layer balloon comprising the innerballoon 30 and the outer balloon 31 can be fluted and wrapped inpreparation for attachment to a catheter shaft. In one embodiment, themulti-layer balloon is fluted and wrapped in preparation for insertioninto another balloon. In another embodiment, the multi-layer balloon isfluted and wrapped in preparation for having another balloon insertedinto a cavity defined by the multi-layer balloon.

The above-disclosed nesting method is particularly suitable for ultrahigh pressure balloons having large neck diameters relative to theirbody size. Further variations to the nesting method are possible suchas, for example, repetition of this process to produce many-layeredballoons, use of non-identically sized or shaped balloons, omission oflubricating layers for certain interfaces, and other suitable methodsand processes.

The above-disclosed method comprising independent formation of an innerballoon and an outer balloon and then nesting the balloons allows for avariety of balloon sizes and shapes at each layer. Therefore, thismethod typically allows for ideal balloon parameters at each layer.However, in some instances, independent formation of balloon layerscould be a slower and more costly process, particularly for balloonswith small necks relative to their bodies. Typically, the body of theballoon is wider than its neck. However, the body of the inner balloonshould still be capable of fitting through the neck of the outerballoon. The body of a balloon can be narrowed by heating, stretching,fluting, and wrapping. The neck of a balloon can possibly be widened byheating and inflating or stretching the balloon radially, but thesemethods are limited. As a result, it is often practical to form balloonsindependently and then nest them to create multi-layer balloons with aballoon body diameter to neck diameter ratio of 4 to 1 or less. Forlarger diameter-to-neck ratios, co-extrusion of some balloon layersmight be preferable.

In the co-extrusion method, as discussed in further detail below withreference to FIGS. 23A through 23D, one avoids the difficulty of nestingballoons. However, the process of co-extrusion limits one's control overthe size and shape of each balloon layer, potentially causing some ofthe problems discussed above, as well as others due to the general lostdesign freedom. In general, co-extrusion is more efficient than nestingin manufacturing larger multi-layer balloons, in the range aboveapproximately 12 mm in diameter.

FIGS. 23A through 23D show one embodiment of forming multi-layerballoons using a co-extrusion method. As shown in FIG. 23A, a threelayer balloon 26 and a two layer balloon 27 are provided. The threelayer balloon 26 preferably has two structural layers 22, 24 and onelubricating layer 23. The two layer balloon 27 preferably has onestructural layer 20 and one lubricating layer 21. In this embodiment,the three layer balloon 26 and the two layer balloon 27 are bothco-extruded. In one embodiment, the structural layers comprise apolyamide such as Nylon 12. In one embodiment, the lubricating layerscomprise 0.0001 to 0.00015 inch high-density polyethylene and/or carbonnanopowder filler.

With reference to FIG. 23B, the three layer balloon 26 is then processedto reduce its diameter and cross-sectional area in a manner similar tothat disclosed above with respect to FIG. 22B of the nesting method.That is, the three layer balloon 26 can be heated and stretched so thatthe diameter and cross-sectional area of the three layer balloon 26 isdecreased, while the length of the balloon 26 is at least partiallyincreased. Heating and stretching the three layer balloon 26 in thismanner typically alters the alignment of the molecules comprising thebody of the balloon 26.

As shown in FIG. 23C, the three layer co-extruded and stretched balloon26 (having a decreased diameter) can then be inserted into the two layerco-extruded balloon 27. In one embodiment, the three layer balloon 26can simply be slid inside the two layer balloon 27. The lubricatinglayer 21 of the two layer balloon 27 (which can be disposed on an innersurface of the two layer balloon 27) facilitates relatively easyinsertion of the three layer balloon 26 into the two layer balloon 27because it reduces friction between the balloons when a structural layer22 of the three layer balloon 26 (which can be disposed on an outersurface of the three layer balloon 26) contacts an inner surface of thetwo layer balloon 27.

With reference to FIG. 23D, the newly-formed five layer multi-layerballoon 25 can be heated, stretched, and inflated such that the innerballoon 26 alters its molecular orientation to an orientation that theinner balloon 26 had prior to the heating and stretching step of FIG.23B (i.e., its original molecular orientation). As a result, themolecular orientation of the inner balloon 26 becomes substantiallysimilar to, or the same as, the molecular orientation of the outerballoon 27 because these balloons had substantially similar, or thesame, molecular orientations after co-extrusion (the step of FIG. 23A)and before drawing down the inner balloon 26 (the step of FIG. 23B).

Other variations of this co-extrusion method are possible such as, forexample, repeating the method steps to create additional balloon layers,using additional co-extruded balloon layers, combining co-extrusion withballoon nesting, using alternative methods to achieve molecularalignment among the balloon layers, and other suitable variations. Asdiscussed above with respect to the nesting method, in some embodimentsof the co-extrusion method, it is important to have balloon layerscomprising the substantially same size and comprised of materials havingsubstantially similar mechanical properties.

Turning now to FIGS. 24A and 24B, before the multi-layer balloon iscomplete, the structural layers of the balloon are typically weldedtogether. Unless the lubricating layers are to be welded as well, thelubricating layers preferably are kept away from a welding zone 40 ofthe multi-layer balloon. When a lubricating layer is not co-extruded, itcan be injected relatively far from the welding zone and thenmechanically dispersed, as shown and described above with respect toFIG. 22E. In FIGS. 24A and 24B, an analogous process is shown for aco-extruded lubricating layer 42 (as opposed to applying a lubricatinglayer in a nesting method for creating multi-layer balloons). In thisembodiment, extrusion of the lubricating layer 42 is periodically haltedto make a lubricating layer-free welding zone 40. In one embodiment,this is accomplished with a diverter valve. Use of the diverter valvecan be adjusted to create a balloon parison of appropriate length. Thestructural layers 41, 43 of the multi-layer co-extruded balloon can thenbe welded together in welding zone 40. Other suitable variations canalso be used to separate the lubricating layers from the welding zones.

FIGS. 25A through 25D show an embodiment of a method for formingtwo-layer balloons with each layer made from tubing that optimizes innerwall stretch for maximized balloon strength. As shown in FIG. 25A, asingle-layer extrusion or tube stock 100 and a single-layer extrusionhaving a slip layer 110 are provided. The single-layer extrusion 100preferably has a single, structural side wall 102. The single layer tubestock having a slip layer 110 preferably has a single, structural sidewall 112 and a slip layer 114 disposed on the inner surface of the sidewall 112. The tube stock with a slip layer 110 has a bonding zone 116 ateach end of the tube stock 110. The bonding zone 116 defines an area ofthe tube stock 110 between the longitudinal ends of the side wall 112and the ends of the slip layer 114. The bonding zone 116 provides anarea free from lubrication, which allows the tube stock without a sliplayer 100 to be bonded with the tube stock with a slip layer 110.

The single-layer tube stock 100 and the single-layer tube stock having aslip layer 110 are preferably formed from tubing that optimizes theinner wall stretch thus providing optimum balloon strength. In oneembodiment, the extrusions 100, 110 may be formed from the same materialand are preferably formed from the same, or a substantially similar,diameter of tube stock such that the degree of biaxial molecularorientation between the balloons 100, 110 is substantially similar. Ifthe tube stocks 100, 110 are composed of the same material, then thediameters of the tube stock should be within about 10% of each other inorder to provide balloon layers having a substantially similar degree ofbiaxial molecular orientation.

In one embodiment, the side walls 102, 112 comprise a polyamide such asNylon 12. In one embodiment, the slip layer 114 comprises a layercomposed of 0.0001 to 0.00015 inch high-density polyethylene and/orcarbon nanopowder filler (i.e., “bucky balls” or graphite nanocarbonparticles).

By way of example, for an 8 mm balloon made from Nylon 12, a tubing sizeof 0.090 inches by 0.056 inches may be used. The slip layer preferablywill, at a minimum, cover a substantial portion of the main body of theballoon comprising the cylindrical portion of the balloon. However, insome applications, the slip layer may extend beyond the body of theballoon to cover at least a portion of the conical section of theballoon.

With reference to FIG. 25B, the single-layer extrusion 100 is thenprocessed to reduce its diameter and cross-sectional area in a mannersimilar to that disclosed above with respect to FIGS. 22B and 23B. Thatis, the single-layer tube stock 100 can be heated and stretched axiallyso that its diameter and cross-sectional area are at least partiallydecreased, while the length of the extrusion 100 is at least partiallyincreased. Heating and axial stretching the single-layer extrusion 100in this manner typically alters the axial alignment of the moleculescomprising the body of the extrusion 100, but induces little or nochange to the radial or circumferential alignment.

As shown in FIG. 25C, the single-layer tube stock 100 (having adecreased diameter) can then be inserted into the single-layer tubestock having a slip layer 110 (having its original, unaltered diameter).In one embodiment, the single-layer extrusion 100 can simply be slidconcentrically inside the single-layer extrusion having a slip layer110. The slip layer 114 of the single-layer tube stock 110 facilitatesrelatively easy insertion of the single-layer tube stock 100 into thesingle layer tube stock having a slip layer 110 because it reducesfriction between the balloons when the side wall 102 of the single-layerextrusion 100 contacts an inner surface of the single-layer extrusionhaving a slip layer 110.

With reference to FIG. 25D, the newly-formed two-layer balloon stock orparison 120 can be heated, stretched, and inflated such that the innertube stock 100 alters its molecular orientation to an orientation thatthe inner tube stock 100 had prior to the heating and stretching step ofFIG. 25B (i.e., its original molecular orientation). As a result, thedegree of biaxial molecular orientation of the inner tube stock 100becomes substantially similar to, or the same as, the degree of biaxialmolecular orientation of the outer tube stock 110 because these balloonshad substantially similar, or the same, molecular orientations at thebeginning of the above-described process (the step of FIG. 25A) andbefore drawing down the inner tube stock 100 (the step of FIG. 25B).

It should be noted that in some applications of the multi-layer balloonsformed using the methods described herein, such as the two-layer parisonas described with reference to FIGS. 25A through 25D, the multi-layerparison does not necessarily have lubricating or slip layers. Forexample, the two-layer parison 120 of FIGS. 25A through 25D can simplycomprise two single-layer extrusions formed from a substantially similarsized tube stock without having a slip layer disposed between the twoside walls of the two-layer balloon stock 120.

Experiment to Test Superiority of Bi-Layer Balloon with Maximized RadialExpansion

An experiment was conducted to test the superiority of a bi-layerballoon with maximized radial expansion. The experiment was performedusing the following three high pressure balloon designs: (1) a bi-layerballoon with both balloons having maximized radial expansion (“BalloonDesign 1”); (2) a bi-layer balloon, produced from telescoping extrusion,with balloons having different expansion ratios (“Balloon Design 2”);and (3) a single layer balloon having a relatively thick wall (“BalloonDesign 3”). Tests were conducted and utilized to provide statisticalproof of certain characteristics of the three high pressure balloondesigns, such as burst pressure, compliance, and fatigue testing.

The results of the experiment indicate that a bi-layer balloon with bothballoons having maximized radial expansion (i.e., Balloon Design 1) hasa 12% greater burst strength and can be subjected to 46% more fatiguecycles than a bi-layer balloon with balloons having different expansionratios (i.e., Balloon Design 2). The results also demonstrate that abi-layer balloon with both balloons having maximized radial expansion(i.e., Balloon Design 1) has a 14% greater burst strength and can besubjected to 68% more fatigue cycles than a single layer balloon havinga thick wall (i.e., Balloon Design 3).

Purpose of the Experiment:

-   -   Create and test multiple variations of plausible, high pressure,        Nylon 12 balloon designs.    -   Challenge the theory that two nested balloons with maximized        expansion ratios is superior to nested balloons with different        expansion ratios and thick single walled balloons.

Tools and Equipment:

-   -   All burst, compliance, and fatigue testing were completed with        the following machines:        -   PT-3070 (Pressure Regulation): IA Asset 620        -   Laser Measurement System: IA Asset 326        -   Temperature Control System: IA Asset 519    -   Wall Thickness Measurement Tool:        -   Mitutoyo Blade Micrometer: IA Asset 173    -   Balloon Blowing Equipment:        -   Balloon Forming Machine: 2210H/110V        -   Computerized Double End Stretcher: CJS-3X12/110V        -   Center Mold: 316061-408        -   Distal End Plug: 502155-35        -   Proximal End Plug: 502155-34

Part Number and Description of the Balloons Used for Patent Testing:

-   -   1. 511023 (Balloon Design 1): Multilayer balloon with both        balloons having maximized radial expansion.        -   a. Inner and outer balloon extrusion part number: 315284-08        -   b. Inner and outer balloon extrusion dimensions:            0.090″×0.056″    -   2. 316085-X1 (Balloon Design 2): Multilayer balloon created from        nested extrusion.        -   a. Inner extrusion part number: 315284-08        -   b. Inner extrusion dimensions: 0.090″×0.056″        -   c. Outer extrusion part number: 315284-X1        -   d. Outer extrusion dimensions: 0.126″×0.092″    -   3. 316085-X2 (Balloon Design 3): Thick, single layer balloon.        -   a. Extrusion part number: 315284-X2        -   b. Extrusion dimensions: 0.124″×0.056″    -   4. 316085-X3: Multilayer balloon created from nested extrusion.        The inner layer extrusion is the same part number as the outer        and is drawn down through a hot die so the outer diameter is        slightly smaller than the inner diameter of the original tubing        size.        -   a. Extrusion part number: 315284-08        -   b. Extrusion dimensions: 0.090″×0.056″

Description of the Testing Requirements:

-   -   1. Burst and Compliance Testing: 10 samples per balloon part        number.        -   While being submerged in 37° C. water, the balloon diameter            is measured and recorded while being stepped in 2 ATM            increments. The pressure is stepped and recorded until the            balloon bursts. Compliance percentage, average burst, and            minimum burst strength (“MBS”) are calculated.    -   2. Fatigue Testing: 10 samples per balloon part number.        -   Once the different balloons have been burst tested, the            least MBS calculated will be used for fatigue testing.        -   Each balloon will undergo cycles from 0 to MBS until the            balloon bursts. The number of cycles will be recorded and an            average will be calculated.

Balloon Development Notes:

-   -   1. Balloon part numbers 511023, 316085-X1, and 316085-X2 were        formed using usual balloon blowing techniques.    -   2. Balloon part number 316085-X3 was not able to be formed.        -   To start the development, the necked down inner tubing was            solely used to form the 8 mm balloon.        -   The logic used was if the inner balloon was not able to be            formed, the nested extrusion will also not be able to be            formed.        -   The extrusion was not able to expand to the walls of the            mold due to an inner expansion ratio of approximately 14:1.

Results:

Balloon Wall Thickness Measurements 316085-X1 (Telescoped 316085-X2316085 & 316086 tubing w/o (Thick (Standard) necking) wall tubing)Double Wall Thickness Measurement (Inches) Burst Sample Number 1 0.00620.0065 0.0068 2 0.0062 0.0063 0.0069 3 0.0064 0.0063 0.0067 4 0.00630.0068 0.0067 5 0.0064 0.0064 0.0069 6 0.0062 0.0070 0.0069 7 0.00630.0065 0.0068 8 0.0064 0.0065 0.0069 9 0.0064 0.0069 0.0068 10  0.00640.0063 0.0068 Average Double Wall 0.0063 0.0066 0.0068 Thickness (In)St. Dev. 0.0001 0.0003 0.0001 % St. Dev. 1.5% 4.0% 1.2% RelativeDifference   0% 3.6% 7.9% Fatigue Sample Number 1 0.0064 0.0066 0.0068 20.0064 0.0065 0.0067 3 0.0064 0.0065 0.0067 4 0.0063 0.0063 0.0068 50.0064 0.0068 0.0069 6 0.0063 0.0066 0.0069 7 0.0065 0.0067 0.0069 80.0064 0.0067 0.0069 9 0.0063 0.0068 0.0068 10  0.0064 0.0067 0.0068Average Double Wall 0.0064 0.0066 0.0068 Thickness (In) St. Dev. 0.00010.0002 0.0001 % St. Dev. 1.0% 2.3% 1.2% Relative Difference   0% 3.8%6.9%

Burst Testing 316085-X1 316085 & (Telescoped 316085-X2 316086 tubing w/o(Thick Burst Sample Number (Standard) necking) wall tubing) 1 40.0439.46 36.09 2 41.38 40.03 35.94 3 40.27 37.85 36.08 4 42.19 36.22 35.945 42.21 34.03 37.37 6 40.03 37.89 35.94 7 43.76 38.12 36.14 8 40.03 37.436.76 9 44.34 34.04 37.85 10  42.75 37.78 35.94 Average Burst (atm)41.70 37.28 36.41 Relative Burst (atm) Standard 35.93 33.52 St. Dev.1.609 2.005 0.690 % St. Dev. 3.9% 5.4% 1.9% K-Factor 5.203 5.203 5.203MBS = fatigue pressure 33.33 26.85 32.81 Relative MBS 33.33 (Standard)25.87 30.22 Minimum (atm) 40.03 34.03 35.94 Maximum (atm) 44.34 40.0337.85

Fatigue Testing 316085 & 316085-X1 316086 (Telescoped 316085-X2 FatigueSample (Nested tubing w/o (Thick (number of cycles at fail) Balloons)necking) wall tubing) Fatigue Pressure (atm) 27 27 27 1 45 41 20 2 71 3732 3 53 57 25 4 134 64 28 5 150 48 13 6 101 67 24 7 81 38 30 8 82 47 329 42 32 32 10  119 41 45 Average Cycle Number 88 47 28 StandardDeviation 37 12  9 % St. Dev. 42.7% 25.1% 30.4% Relative Average 88(Standard) 49 30

Description of Balloon Failure:

-   -   511023:        -   12% higher burst and 46% more fatigue cycles than 316085-X1.        -   14% higher burst and 68% more fatigue cycles than 316085-X2.        -   The superiority of the nested balloons is due to both            balloons having the identical inner and outer expansion            ratios.        -   The stresses caused by inflation and deflation are the same            for each balloon when both have the same inner and outer            expansion ratios. When the stresses are the same, the            balloons will burst at the same time which ensures maximized            burst strength.    -   316085-X1:        -   The lower burst strength and fatigue cycles of this balloon            are due to the nested tubing having different expansion            ratios.        -   The outer extrusion has a medium expansion ratio of 2.9:1            while the inner extrusion has 4.4:1.        -   Due to the expansion ratio difference, the balloon layers            are undergoing different stresses while being inflated and            deflated.        -   During the burst test, two distinctive “pops” can be heard.            The first rupture is the inner balloon and the second            rupture is the outer balloon.        -   Due to the outer balloons lower expansion ratio, it can            withstand more pressure and fatigue cycles than the inner            balloon.        -   Once the inner balloon bursts, the second balloon            immediately ruptures because the pressure is no longer            contained by two layers.    -   318085-X2:        -   The lower burst strength and fatigue cycles of the thick            walled balloon are caused by the same concept of a balloon            with nested tubing having different expansion ratios.        -   The outer expansion ratio is 2.6:1 while the inner expansion            ratio is 5.75:1. The inner diameter has to expand            approximately 2.2 times farther than the outer diameter.        -   During the burst and fatigue testing, the inner surface of            the balloon begins to fracture before the outer surface.            This is due to the inner surface reaching its maximum            expansion while the outer surface proceeds to grow.        -   Once a fracture begins on the inner surface of the balloon,            it quickly tears through the entire wall of the balloon            causing premature bursts.

According to the results of the experiment as set forth above, it isconcluded that a bi-layer balloon constructed with two balloons with thesame expansion ratios (i.e., Balloon Design 1) proves to have superiorburst strength and cycle fatigue resistance when compared to a bi-layerballoon with balloons having different expansion ratios (i.e., BalloonDesign 2) and to a single layer balloon having a thick wall (i.e.,Balloon Design 3).

Although this invention has been disclosed in the context of certainpreferred embodiments and examples, it will be understood by thoseskilled in the art that the present invention extends beyond thespecifically disclosed embodiments to other alternative embodimentsand/or uses of the invention and obvious modifications and equivalentsthereof. In addition, while a number of variations of the invention havebeen shown and described in detail, other modifications, which arewithin the scope of this invention, will be readily apparent to those ofskill in the art based upon this disclosure. It is also contemplatedthat various combinations or sub-combinations of the specific featuresand aspects of the embodiments may be made and still fall within thescope of the invention. Accordingly, it should be understood thatvarious features and aspects of the disclosed embodiments can becombined with or substituted for one another in order to form varyingmodes of the disclosed invention. Thus, it is intended that the scope ofthe present invention herein disclosed should not be limited by theparticular disclosed embodiments described above, but should bedetermined only by a fair reading of the claims.

What is claimed is:
 1. A method of making a tubular balloon stock foruse in fabricating a balloon catheter, comprising the steps of:providing a first tubular section comprising a first layer and a secondtubular section comprising a second layer; determining the stretchrequired to optimize the strength at the inner surface of each layer;stretching each layer to within approximately 15% of the determinedstretch for each layer; and positioning the first layer concentricallywithin the second layer to form the tubular balloon stock.
 2. A methodof making a tubular balloon stock as in claim 1, wherein the stretchingstep is accomplished before the positioning step.
 3. A method of makinga tubular balloon stock as in claim 1, wherein the stretching step isaccomplished after the positioning step.
 4. A method of making a tubularballoon stock as in claim 1, further comprising fluting the firsttubular section.
 5. A method of making a tubular balloon stock as inclaim 4, further comprising wrapping the first tubular section.
 6. Amethod of making a tubular balloon stock as in claim 5, wherein thefluting and wrapping steps are accomplished before the positioning step.7. A method of making a tubular balloon stock as in claim 1, wherein aradially inwardly facing surface of the second layer is provided with aslip layer.
 8. A method of making a tubular balloon stock as in claim 7,wherein the slip layer comprises carbon nanoparticles.
 9. A method ofmaking a tubular balloon stock as in claim 1, wherein at least onetubular section comprises nylon.
 10. A method of making a tubularballoon stock as in claim 1, wherein the first and second tubularsections comprise nylon.
 11. A method of making a tubular balloon stockas in claim 1, wherein the stretching step comprises stretching eachlayer to within approximately 10% of the determined stretch for eachlayer.
 12. A method of making a tubular balloon stock as in claim 1,wherein the stretching step comprises stretching each layer to withinapproximately 5% of the determined stretch for each layer.
 13. A methodof making a tubular balloon stock as in claim 1, wherein the first layerand second layer fail at approximately the same pressure when a pressureis applied to the balloon.
 14. A method of making a tubular balloonstock as in claim 1, wherein the first and second layers are configuredto withstand at least about 40 atmospheres of applied pressure.
 15. Amethod of making a tubular balloon stock as in claim 1, wherein thefirst and second layers are configured to withstand at least about 50atmospheres of applied pressure.
 16. A method of making a tubularballoon stock as in claim 1, wherein the first and second tubularsections have substantially the same inner diameter and substantiallythe same outer diameter.